This invention relates to a lens driving device and, in particular, to an autofocus lens driving device for use in a portable compact camera.
The portable compact camera is mounted in a camera-equipped cellular mobile phone. The portable compact camera is provided with the autofocus lens driving device. Previously, various autofocus lens driving devices have been proposed.
By way of illustration, Japanese Unexamined Patent Application Publication No. 2005-128392 (JP-A-2005-128392) (which will be also called a first patent document) discloses a lens driving device which is excellent in shock resistance. The lens driving device disclosed in the first patent document comprises a substantially cylindrical shaped yoke, a base in which the yoke is mounted, a lens supporting body (a lens holder) disposed in the yoke at an inner side thereof, and a pair of leaf springs mounted on the lens supporting body. The pair of leaf springs is called a front-side spring and a rear-side spring. The front-side spring is disposed to the lens supporting body at a front side (one side) in a direction of an optical direction of a lens therein while the rear-side spring is disposed to the lens supporting body at a rear side (another side) in the direction of the optical direction of the lens therein. The yoke has a substantially U-shape in cross section. A magnet (a permanent magnet) is disposed in the interior of the U-shape and a coil is disposed in the interior of the U-shape at an inner side of the magnet. The magnet is fixed to the base via a rear spacer. The coil is fixed to an outer portion of the lens supporting body. The outer portion of the lens supporting body is inserted in the interior of the yoke. Thereby, the outer portion of the lens supporting body moves in a gap in the U-shape of the yoke. The yoke is fixed to the base at an inner side. The rear-side spring has an outer end portion which is fixed between a base portion of the base and a rear spacer.
In the lens driving device disclosed in the first patent document, the lens supporting body has a substantially cylindrical shape and has an interior in which a lens (a lens assembly, a lens barrel) is accommodated. The lens supporting body is movably mounted at the inner side of the yoke along the optical direction. The front-side spring comprises a substantially ring-shaped leaf spring which comprises an inner end portion and an outer end portion. The inner end portion of the front-side spring is sandwiched between the lens supporting body and a cap to be fixed. The front spacer is inserted between the outer end portion of the front-side spring and the yoke. The rear-side spring has a substantially same shape to that of the front-side spring and comprises a substantially ring-shaped leaf spring. The rear-side spring has an inner end portion which is fixed to a rear end of the lens supporting body. The rear-side spring has an outer end portion which is fixed between the base portion of the base and the rear spacer.
In the lens driving device disclosed in the first patent document, the coil has a cylindrical shape and the magnet also has a cylindrical shape. Specifically, the magnet comprises four magnet pieces each having an arc shape and has a cylindrical shape as a whole.
In addition, Japanese Unexamined Patent Application Publication No. 2006-293243 (JP-A-2006-293243) (which will be also called a second patent document) discloses a lens driving device which is capable of smoothly driving a lens and also accurately turning the optical axis direction of the lens to a photographing direction even when a camera is used with the photographing direction inclined to a vertical direction. The lens driving device disclosed in the second patent document comprises a holder (a lens holder) holding a lens, a ring-shaped driving coil mounted to the holder, a magnetic field generating arrangement disposed in a fixed member side and applying a radial driving magnetic field to the coil, a first spring member (a front-side spring) coupling an upper side of the holder with an upper side of the fixed member, and a second spring member (a rear-side spring) coupling a lower side of the holder with a lower side of the fixed member. Herein, in the manner which is similar to the above-mentioned first patent document, the front-side spring has an inner end portion which is sandwiched between the lens holder and a cap to be fixed.
In the second patent document, the magnetic field generating arrangement comprises a combination of a cylindrical-shaped magnet disposed to an outer side of the ring-shaped coil and a U-shaped cylindrical yoke consisting of a magnetic substance such as soft-iron.
Furthermore, Japanese Unexamined Patent Application Publication No. 2006-258969 (JP-A 2006-258969) (which will be also called a third patent document) discloses a camera-equipped cellular mobile phone which is capable of shorting a time required for automatic focusing. The camera-equipped cellular mobile phone (a lens driving device) disclosed in the third patent document a holder (a lens holder) including a tubular portion having an end to which the lens (the lens assembly, the lens barrel) is mounted, a driving coil fixed to the holder (the lens holder) so as to position around the tubular portion of the holder, a yoke comprising a permanent magnet opposite to the driving coil, a pair of leaf springs disposed to both sides of the tubular portion of the holder (the lens holder) in the direction of the optical axis. The pair of leaf springs supports the holder (the lens holder) in the direction of the optical axis shiftably so as to position the holder (the lens holder) in a radial direction. By energizing the driving coil, it is possible to positionally adjust the lens (the lens assembly) in the direction of the optical axis according to interaction between a magnetic field of the permanent magnet and a magnetic field due to an electric current flowing through the driving coil. In the pair of leaf springs, the one is called an upper leaf spring (a front-side spring), the other is called a lower leaf spring (a rear-side spring). The upper leaf spring (the front-side spring) has an inner end portion which is sandwiched between an upper end (a front end) of the holder and a stopper to be fitted to the holder (the lens holder).
In the third patent document, the driving coil has a cylindrical shape and a plurality of permanent magnets are disposed to an inner surface of an outer tubular portion of the yoke with a space left between the driving coil and it. Each of the permanent magnets has an arc shape.
Inasmuch as, in each of structures disclosed in the above-mentioned first through third patent documents, the driving coil has the cylindrical shape, the magnet (the permanent magnet) has the cylindrical shape (the arc shape). However, it is difficult to manufacture such a magnet (permanent magnet) having the cylindrical shape (the arc shape) and the magnet (the permanent magnet) is expensive. As a result, it is disadvantageous in that the lens driving device is also expensive.
In addition, Japanese Unexamined Patent Application Publication No. 2007-121853 (JP-A-2007-121853) (which will be also called a fourth patent document) discloses an image pickup apparatus in which thickness of a magnet in a direction orthogonal to an optical axis of an image pickup lens is different according to positions in a circumferential direction. In the fourth patent document, as modification of an embodiment, the magnet comprises four first divided portions each having a first thickness and having a rectangular shape in a cross section in a plane orthogonal to the optical direction and four second divided portions each having a second thickness and having a rectangular shape in a plane orthogonal to the optical direction that are alternately disposed in a circumferential direction. The first thickness is thicker than the second thickness.
The fourth patent document uses, as the permanent magnet, a substantially flat shape one. However, inasmuch as a magnetic circuit in the fourth patent document comprises the magnets and the yoke alone, it is disadvantageous in that magnetic efficiency thereof is low.